This invention provides new pharmaceutically useful compounds that are prodrugs of compounds known to be inhibitors of the serine protease plasma kallikrein (PK). Upon administration to a subject having a disease or condition that can be treated with a plasma kallikrein inhibitor, they are converted in vivo into the active compounds, and therefore have valuable properties as pharmaceutical agents. This invention also provides new compounds that are especially useful as inhibitors of plasma kallikrein, in that they exhibit both high potency inhibiting this enzyme, and also exhibit pharmacologically desirable properties when administered in vivo.